Basically, magnitude just means how big it is.
If you have a vector (in, say, three dimensions) [tex]\vec x = (x_1, x_2, x_3)[/tex] then the magnitude of that vector is given by (Pythagoras): [tex]|\vec x| = \sqrt{x_1^2 + x_2^2 + x_3^2}[/tex].

still not very clear how it is related to distance and displacement........then wt is magnitude of displacement.

Displacement is a vector quantity defined as the change in position - from an initial point i to a final point f. Its magnitude is the length of the straight line between i and f and its direction is from i to f. The actual path taken is irrelevant.
Distance is a scaler quantity defined as the path length, i.e., it does depend on the actual path taken. Distance only equals the magnitude of the displacement for straight line paths.
Hope this hepls

Displacement is a vector quantity. It has direction and magnitude. The absolute quantity of vector is its magnitude.
You can use the formula posted by CompuChip to calculate it
Sorry if I said something wrong

Simpler still, magnitude is the size or length (always positive or zero) of a vector, independent of the vector direction. "Displacement" in Newtonian physics, is a vector, and has two elements, a signed length (positive, negative, or zero) and a direction.